Jackie’s flat, East Sheen, May 2013. Jackie and Justin have been together, on and off, for more than 20 years.

My brother, Justin, is not a straightforward person,” says photographer Louis Quail, “His otherness is very apparent. I have seen people recoil from him on the bus.” Justin was diagnosed with paranoid schizophrenia when he was 20. Now 57, his adult life has been defined to a great degree by his condition and by people’s reactions to it, whether the public, the authorities or his doctors. “I wanted to challenge the stigma of mental illness,” says Louis, “and one way to do that is to show the light and shade in his life, the small details as well as the dramas.”

Big Brother is a moving, sometimes disturbing and often very funny ode to his schizophrenic sibling, whose fertile creative imagination and extraordinary resilience are evident throughout. It begins with a series of Justin’s watercolours and pencil sketches of birds as well as a handwritten inventory of the 56 species that he observed as a 14-year-old birdwatcher over two days in May 1973. The sense of adolescent curiosity bordering on obsession is dramatically disrupted by the first photograph, a portrait of the 53-year-old Justin sitting in his kitchen. His hands cover his face as if to shut out the clamour and confusion of the world around him. Wisps of smoke curl above his head from the cigarette he is clasping between his fingers as if for dear life. He seems anguished, all at sea.

On the opposite page, Louis explains the context for the portrait. “Last night Justin was arrested for ‘criminal damage to a glass’ and I am here, today, in my capacity as ‘appropriate adult’, trying to understand the facts behind the charge, which on the face of it seem slightly ludicrous.” He also writes: “I Fought the Law is apparently one of the songs that Justin has been singing at the top of his voice throughout the night. When he gets agitated he’s prone to such outbursts, and this morning he has good reason.”

Justin at home in Mortlake, March 2012. ‘When Justin gets agitated he’s prone to outbursts,’ says Louis. ‘Last night he was arrested.’

In anger, Justin had kicked over a table in a pub, causing a glass to smash. All the dramas and misunderstandings that dog Justin’s life are distilled into that single, illuminating vignette, while the anguished portrait conveys the psychic cost of living such an unruly and, in its own peculiar way, heroically defiant life. The book is punctuated by similar moments in which Justin’s impulsive behaviour inevitably lands him in trouble. But it also observes his moments of calmness and creativity – as well as painting and drawing, he writes starkly self-insightful poetry about himself. “I wanted to present a series of surprises that catch you off guard and make you question your assumptions,” elaborates Louis Quail, “So, the book begins with a troubled individual who also happens to be a keen birdwatcher. Suddenly, he has a girlfriend, and, not only that, but they are going on holiday together. Hopefully, it unfolds the way a novel unfolds, with a series of doors opening on to a life.”

Most of the pictures of Justin were taken in and around Richmond and Twickenham, where he has lived since giving up what Louis calls his “hippy hobo lifestyle”. In his younger years, after the break-up of his parent’s marriage, he lived for a while in a squat, often taking off for weeks on end to wander the country in pursuit of his hobby. Once, aged 15, he hitchhiked to the Outer Hebrides, returning immediately after he spotted the rare bird he was seeking. Around this time, too, he began experimenting with LSD and magic mushrooms, which inevitably exacerbated his problems. He remains philosophical about the experience, saying simply: “I got into drugs, mysticism and make‑believe”.

These days Justin embraces more calming pursuits. Among the surprises is a portrait of Justin meditating, eyes closed, in a classic cross-legged yoga pose. In another portrait, he seems almost beatific, wrapped in a blanket in the woods. Louis also captures several moments of tenderness between Justin and his longtime girlfriend, Jackie, though their relationship tends towards the turbulent. Birdwatching, too, remains a constant in Justin’s life, an activity encouraged by his parents back when things were almost normal. The birds, he tells his brother, “reflect the earth back in you”. In a poem, The Thrilling of the Teal, he writes with a seemingly acute understanding of his condition: “Peace and quiet may save me/ but for the blustering winds against my odds.”

Justin meditating at ‘the den’, a disused garage near his art class in Roehampton.

Chaos, though, is never far away and can erupt with a tidal force that is as unexpected as it is disruptive. It is, more often than not, says Louis, “the chaos of mental illness meeting the increasingly rigid structures of law and officialdom”. Big Brother, as its double-edged title suggests, is also a narrative that lays bare in the most vividly personal way how austerity cuts in policing, health and housing have affected the welfare of the vulnerable in often unforeseen ways. “What I have witnessed over the last few years is a kind of zero tolerance approach to Justin’s behaviour, “ says Louis, sounding exasperated. “He has a temper and he can be unpredictable, but, more and more, relatively minor issues are treated in terms of the worst case scenario, whereas, in the past, there would have been a more specialised, and thus sympathetic, understanding of his issues.”

In one passage in the book, Louis details some examples of the worst case scenario approach: “Justin visits a care worker at home and becomes a ‘stalker’; he locks his cleaners in his flat and becomes a ‘hostage taker’.” What we are witnessing, he suggests, is the increasing criminalisation of the mentally ill, the labels society places on them become ever more extreme and judgmental. “When I look at Justin’s medical and forensic notes,” he writes, “especially the regular risk assessments, I begin to wonder if I’m actually reading about the same person I know.”

To complicate matters, Justin’s girlfriend, Jackie, is an alcoholic. They have been together, on and off, since 1995 and make for quite a pair. Alcohol defines Jackie’s life and their co-dependency is obvious and, at times, a source of further anxiety to Justin. Yet, as Louis’s photographs of the couple show, they are devoted to each other – and to their shared spirituality. It is best summed up by Justin, who regularly chants to Jesus, but also says: “It doesn’t matter if you believe in God or not, it’s therapeutic.”

Justin, July 2014. ‘He’s troubled, but he’s also a painter, a birdwatcher, and writes poetry.’

The book unflinchingly traces their relationship though a relatively uneventful holiday in the seaside town of Port Stewart, Northern Ireland, and on through a particularly grim period precipitated by Jackie’s increasingly heavy drinking and erratic behaviour. On the page, Jackie’s physical decline is rapid and the images of her at her most vulnerable are distressing. When I bring up the ethics of photographing the vulnerable in extremis, Louis is pragmatic. “In the back of your mind as a photographer there is always an awareness of the risk your subjects may not have a good experience of the process, but I thought that was a risk worth taking. Both Justin and Jackie have responsibility to give consent. And, with Justin in particular, I know what worries or disturbs him. He had concerns like everyone else, but he agreed to the collaboration and seems to enjoy the attention the book has brought.”

Against all the odds, Justin and Jackie’s relationship endures. “At its heart, this is a love story,” writes Louis in the concluding chapter, opposite a portrait of the couple holding hands against a wintry sky. It is also a deeply personal evocation of an individual’s struggle, which by extension illuminates the lives of countless vulnerable people who may not possess Justin’s resilience and defiance. As Louis mentions more than once, one in four of us will at some time in our lives grapple with mental illness, yet it remains, despite the efforts of celebrity advocates such as Ruby Wax and Stephen Fry, a subject shrouded in prejudice and misunderstanding. “From my experience,” he says, “being ignored is worse than being intruded on. People who are marginalised want their stories told.”

Big Brother by Louis Quail is published by Dewi Lewis (£35). To order a copy for £29.75 go to guardianbookshop.com or call 0330 333 6846